The power to knock down prison walls. This is the power of Ali's legacy and history.

Muhammad Ali turned 70 on Tuesday, and the three-time heavyweight champion who doubled as the most famous draft resistor in U.S. history remains larger than life in the American mind, despite being ravaged by Parkinson's disease. Two years ago, on a visit to Louisville, Ky., I was reminded why.

In a cab on the way to the Muhammad Ali Center downtown, I saw that my driver had a Vietnam Veterans of America patch on display by his license. I asked him about his experience in Southeast Asia, and he started talking a mile a minute about his time "in country," how his "happiest days" were being a sniper in Vietnam. He even said: "You might not know this, being from Washington, D.C., but the most dangerous animal to hunt is man." He then described the task in detail. He wanted to make sure I left his cab fully aware of his pride, patriotism and unwavering belief in the duty of going to war when country called.

I didn't engage the driver in a debate about Vietnam or U.S. imperialism, but given my reason for being in Louisville, I couldn't resist one question. I asked: "What do you think about Muhammad Ali? He opposed the war in Vietnam. He called it an illegal war aimed at increasing oppression throughout the globe.

"Now you're in a city where there is a Muhammad Ali Street and you're taking me to the Muhammad Ali Center. Does that bother you?"

Without skipping a beat, my cabdriver said, "Well, you have to love Ali."

I asked why, and this produced a pause. "He believed what he believed and no one could tell him different. He stuck to his own guns and, well, you gotta love Ali."

In recent years there has been a cottage industry in Ali revisionism that has been aimed at diminishing his relevance, courage and impact. Ali has been made safe for public consumption. When appearing in public, he's presented as little more than a muted symbol of a troubled past. But in the answer I heard from the cabdriver, I think we can see why it's been so difficult to erase his real legacy

Muhammad Ali's brilliance was not that he was an antiwar prophet. He wasn't Malcolm X in boxing gloves, debating foreign policy between rounds, jabbing his hands and then saying, "So how about that Cuban missile crisis." But unlike the Ivy League advisors who made up the "best and the brightest" in power in those days, Ali understood that there was justice and injustice, right and wrong. He knew that not taking a stand could be as political a statement as taking one.

Ali, strictly in boxing alone, was an all-time great. He was an Olympic gold medalist at 18, the sport's first three-time heavyweight champion and the participant in multiple matches that contend for the title of Fight of the Century. But it was his highly improvisational political courage that transformed him into a legend.

Ali's refusal to fight in Vietnam was front-page news all over the world. In June 1967, he was found guilty of draft evasion by an all-white jury in Houston. The typical sentence was 18 months. Ali received five years and the confiscation of his passport. He immediately appealed, and his sentence was eventually overturned by the U.S. Supreme Court. Ali, undefeated and untouched at this point in his career, was stripped of his title for refusing to serve in the military, beginning a 3 1/2-year exile from the ring.

One group that deeply understood the significance of Ali's stand was Congress. The day of his conviction, the House voted 337 to 29 to extend the draft four more years. It also voted 385 to 19 to make it a federal crime to desecrate the flag.

By 1968, Ali was out on bail — with no boxing ring to call home. But he was never more active, because a young generation of blacks and whites wanted to hear what he had to say. And Ali obliged. In 1968, he spoke at 200 campuses. In one speech, brimming with confidence — as if the might of the U.S. government were no more menacing than a club fighter — Ali said, "I'm expected to go overseas to help free people in South Vietnam and at the same time my people here are being brutalized; hell no! I would like to say to those of you who think I have lost so much: I have gained everything. I have peace of heart; I have a clear, free conscience. And I am proud. I wake up happy, I go to bed happy, and if I go to jail, I'll go to jail happy."

The significance of what this meant to people around the globe cannot be overstated. Even in extreme isolation in an island prison, Ali's courage reached a former boxer turned political prisoner named Nelson Mandela. After his release, Mandela said: "Ali's struggle made him an international hero. His stand against racism and war could not be kept outside the prison walls."

The power to knock down prison walls. This is the power of Ali's legacy and history. It's a history worth knowing, not least of all because questions of racism and war, tragically, aren't questions resigned to history.

Dave Zirin's most recent book is "The John Carlos Story: The Sports Moment that Changed the World."